The present invention relates to the production of hydrogen and oxygen by photo-assisted electrolysis of water using solar radiation.
It is well known that the direct decomposition of water is possible at semiconductor electrodes in a photoelectrochemical cell. In such a cell, the p-n dry junction is replaced by a p-electrolyte-n junction. Electron-hole pairs are generated by the absorption of light, such as sunlight, in both semiconductor electrodes and, after charge separation due to an energy barrier, the charge carriers take part in electrochemical reactions. At the p-type photocathode, electrons combine with H.sup.+ ions or water to produce hydrogen. At the n-type photoanode, electron "holes" or "positive" charges, combine with OH.sup.- ions or water to produce oxygen. The net effect is decomposition of water in the electrolyte, with production of gaseous hydrogen at the photocathode and gaseous oxygen at the photoanode. An externally induced electrical bias in the form of an imposed voltage differential between the photoanode and photocathode accelerates the photochemical effect.
Sufficient energy is available in normal sunlight to make theoretically possible photoelectrochemical cells in which the energy value of the produced hydrogen and oxygen would substantially exceed the energy "costs" of building and operating the cells. The result would be the availability of virtually unlimited quantities of relatively low cost, non-polluting, energy.
Hydrogen is a very versatile raw material. It is, for example, a most desirable source of fuel and energy due to the clean and non-toxic nature of its combustion products. In addition, it is used, for example, in the fertilizer, metallurgical and electrochemical industries. Similarly, oxygen has wide utility as a chemical feedstock and, if produced very cheaply, or as an essentially free by-product of hydrogen production, could also be used as a high temperature combustion promoter, which would make usable as clean burning fuels a wide range of poorly combustible materials.
Efficient solar energy conversion of water to hydrogen and oxygen therefore could provide a major new, nondepletable, energy source, as well as one which would be free of the environmental pollution problems associated with hydrocarbon fuels.
The rapid increase in costs of conventional fuels, particularly hydrocarbons, in recent years, as well as increasing concern over the environmental effects of utilizing hydrocarbon fuels, has prompted increased research and development efforts in the area of improving catalysts, methods and apparatus for photoelectrolytic decomposition of water to produce pure hydrogen and oxygen. Most previous attempts at photoelectrolysis of water have utilized high band-gap semiconductors, such as titanium dioxide (TiO.sub.2) with a band-gap of about 3.2 eV or gallium phosphide (GaP) with a band-gap of about 2.2 eV, as electrode materials. Although these semiconductors are relatively stable, they are able to absorb only a very small fraction of the solar energy falling on the electrodes because their high band-gaps require relatively more energy to excite or displace electrons in the materials to the point where they will be available for electrolytic reactions. As a result, these materials demonstrate only very small efficiencies for solar energy conversion. They also are relatively high cost due to their rare element components and because they do not have wide industrial applications to contribute to large scale manufacturing economies.
It has been suggested that p- and n-type silicon be utilized as the electrode materials in photoelectrochemical cells. Silicon has a relatively low band-gap of about 1:1 eV and is much lower in cost than prior art electrode materials, because of its inexpensive starting materials and because it is widely used in other industrial applications. Nevertheless, previous attempts to produce a silicon electrode photochemical cell have been disappointing. At least two separate problems have plagued researchers. First, the efficiency for hydrogen production at the cathode has been quite low, on the order of only 1/2 of 1% of the available light energy on the cathode being utilized for hydrogen production. When the photocathode has been plated with high cost catalytic materials, such as platinum, efficiency may be increased to about 10%, but at greatly increased manufacturing cost, due to the high price of platinum or similar catalytic materials. Secondly, use of silicon type photoanodes has been hindered by the tendency of such anodes to form an insulating silica film, which very quickly reduces the efficiency of the anode.
It is, accordingly, the primary object of the present invention to product a photochemical cell for electrolysis of water, which cell is more efficient, more stable and lower cost than cells previously known.
A further object is to provide a p-silicon type photocathode for use in photo-assisted electrolysis of water, which photocathode is both cheaper to manufacture and more efficient for hydrogen production than those previously known, together with a novel method of making the same.
A further object is to provide an improved n-silicon photoanode for use in photo-assisted electrolysis of water, which photoanode is less expensive and more stable than those previously known, together with a novel method of making the same.